We evaluated the role of left atrial appendage (LAA) in the left atrial (LA) reservoir function by assessing the changes in LA flow dynamics after LAA clamping during cardiac surgery. The subjects were 8 patients who had undergone coronary artery bypass grafting (CABG) and 7 who had undergone mitral valvular surgery due to mitral regurgitation. We recorded transmitral, pulmonary venous and LAA flow velocity patterns by intraoperative transesophageal pulsed Doppler echocardiography, monitoring LA pressure before and 5 minutes after LAA clamping. The maximal LAA area was significantly greater, and the peak late diastolic LAA emptying flow velocity was significantly lower before LAA clamping in the mitral regurgitation group than in the CABG group. In both groups, the peak early and late diastolic transmitral and pulmonary venous flow velocities significantly increased, and the peak second systolic pulmonary flow velocity significantly decreased during LAA clamping. There were no significant changes in heart rate and systemic systolic blood pressure during LAA clamping, whereas mean LA pressure and maximal LA dimension significantly increased in both the groups. The LA pressure-volume relation during ventricular systole shifted upward and to the left during LAA clamping, and the slope was steeper in the MR group than in the CABG group. We conclude that the LAA is more compliant than the LA main chamber, and plays an important role in LA reservoir function in the presence of LA pressure and/or volume overload.